This invention relates to an improved vacuum interrupter and more particularly, a vacuum chamber of a vacuum interruptor provided with electrodes for high voltage use.
It is well known that in a vacuum interrupter, current is interrupted by separating a pair of separable electrodes under high vacuum condition of at least below 10.sup.-4 Torr possessing excellent insulating property and arc extinguish property.
FIG. 1 shows a conventional vacuum interrupter, in which open ends of an insulating cylinder 1 are sealed by end plates 2 and 3 to define an air-tight chamber. The air in the chamber is exhausted to creat a vacuum condition of below 10.sup.-4 Torr. In the vacuum chamber, a stationary rod 5 supporting a stationary electrode 4 is supported by the end plate 2 and a movable electrode 6 is disposed to oppose the stationary electrode 4. The movable electrode 6 is secured to a movable rod 7 connected to operating means, not shown, and a metal bellows 8 is air-tightly connected to the movable rod 7 at its one end and to the end plate 3 at the other end so that the vacuum interrupter operates in the chamber maintained at a proper vacuum condition.
An electrostatic shield 9 is secured to the cylinder 1 so as to surround the electrodes 4 and 6 in the vacuum chamber. The shield 9 serves to prevent the lowering of the insulating strength of the inner surface of the cylinder 1 caused by the deposition of metal vapor generated from the electrode due to arc created at the time of the current interruption.
In such vacuum interrupter, the electrodes 4 and 6 are in contact with each other at the closed state and when the movable rod 7 is moved downwardly (in FIG. 1) by the action of the operating means, the movable electrode 6 separates from the stationary electrode 4 and then arc is created across both electrodes. The arc is sustained by metal vapor generated from a cathode, for example, electrode 6, and when the current decreases to zero point, the generation of the metal vapor stops. Thus, it becomes impossible to sustain the arc and the circuit is interrupted.
When a large current is interrupted, unstable arc is generated across the electrodes 4 and 6 by the interaction between a magnetic field generated by the arc itself and a magnetic field created by an external electric circuit. For this reason, the arc moves across the electrode surface towards the both ends or the periphery of the electrode and overheats locally the ends or the periphery, and since a large quantity of metal vapor is evolved, the degree of the vacuum in the chamber is lowered and the interrupting capability is also lowered. It is considered that these adverse phenomena are caused for the reason that the metal vapor or ionized metal vapor escapes outwardly of the electrodes and ions required for sutaining the arc becomes insufficient during the arc generation, thereby causing unstable the arc.
In order to prevent such adverse phenomena, it has been well known to apply a magnetic field to the electrode surface, and actually, as examples for applying the magnetic field in the prior art, (1) a coil 100 is disposed around the outer periphery of the insulating cylinder 1 of the vacuum interrupter, and the current to be interrupted is passed through the coil 100 for generating the magnetic field in a direction vertical to the electrode surface (FIG. 1), and (2) the structure of the electrode is reformed so as to create a strong axial magnetic field by the electrode itself as shown in FIGS. 2(a) through 2(c) without enlarging the vacuum interrupter. The concrete construction of the electrode shown in FIGS. 2(a) through 2(c) is described hereafter with reference to FIGS. 4(a) and 4(b).
However, in the example (1), since the coil 100 is positioned considerably apart from the electrodes 4 and 6, it is difficult to generate sufficiently strong axial magnetic field on the electrode surface and in order to obtain an effective magnetic field the coil must be enlarged and the whole structure of the vacuum interrupter is also enlarged. On the other hand, in the example (2), it is not necessary to enlarge the whole structure of the vacuum interrupter as in the example (1), but in a vacuum interrupter for high voltage use, in order to apply a strong axial magnetic field it is required to sufficiently separate the electrodes 4 and 6.